Disc Valves

Disc Valve is a movable obstruction inside the stationary body that adjustably restricts flow through the valve. Although traditionally disc-shaped, discs come in various shapes. Depending on the type of valve, a disc can move linearly inside a valve, or rotate on the stem , or rotate on a hinge or trunnion

(as in a 

check valve ). A ball

is a round valve member with one or more paths between ports passing through it. By rotating the ball, flow can be directed between different ports. 

Ball valves

use spherical rotors with a cylindrical hole drilled as a fluid passage. 

Plug valves

use cylindrical or conically tapered rotors called 

plugs . [ambiguous]

Other round shapes for rotors are possible as well in 

rotor valves , as long as the rotor can be turned inside the valve body. However not all round or spherical discs are rotors; for example, a ball check valve

uses the ball to block reverse flow, but is not a rotor because operating the valve does not involve rotation of the ball.